This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to optimizing the mechanical configuration of a disc drive to improve operational performance.
Disc drives are used as primary data storage devices in modem computer systems and networks. A typical disc drive comprises one or more rigid magnetic storage discs which are journaled about a rotary hub of a spindle motor to form a disc stack. An array of read/write transducing heads are supported adjacent the disc stack by an actuator to transfer data between tracks of the discs and a host computer in which the disc drive is mounted.
Conventional actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from an actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of a magnetic circuit comprising one or more permanent magnets and magnetically permeable pole pieces. When current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces.
The control of the position of the heads is typically achieved with a closed loop servo system such as disclosed in U.S. Pat. No. 5,262,907 entitled HARD DISC DRIVE WITH IMPROVED SERVO SYSTEM issued Nov. 16, 1993 to Duffy et al. (Duffy""907), and assigned to the assignee of the present invention. A typical servo system utilizes servo information (written to the discs during the disc drive manufacturing process) to detect and control the position of the heads through the generation of a position error signal (PES) which is indicative of the position of the head with respect to a selected track. The PES is generated by the servo system by comparing the relative signal strengths of burst signals generated from precisely located magnetized servo fields in the servo information on the disc surface.
The servo system primarily operates in one of two selectable modes: seeking and track following. A seek operation entails moving a selected head from an initial track to a destination track on the associated disc surface through the initial acceleration and subsequent deceleration of the head away from the initial track and toward the destination track. A velocity control approach is used whereby the velocity of the head is repeatedly estimated (based on measured position) and compared to a velocity profile defining a desired velocity trajectory for the seek. Corrections to the amount of current applied to the coil during the seek are made in relation to the difference between the estimated velocity and the desired velocity.
At such time that the head reaches a predetermined distance away from the destination track (such as one track away), the servo system transitions to a settling mode wherein the head is settled onto the destination track. Thereafter, the servo system enters a track following mode of operation wherein the head is caused to follow the destination track until the next seek operation is performed.
Disc drive designs thus typically use proximate time optimal control with a velocity profile to control a selected head during a seek, a state estimator based controller with relatively slow integration to settle the head onto the destination track, and the same state estimator based controller with relatively fast integration for track following.
Conventional disc drive designers have employed ball bearing cartridges for journaling the actuator assembly about the pivot point. These bearing assemblies are subject to very rapid, repetitive movements of the actuator arm about the pivot point as the heads are radially moved from track to track. The precision required by the rotation of the actuator arm about the bearing assembly has significantly increased as the storage capacity of modem disc drives continues to expand.
The precision of seeking and track following operations is dependent upon the performance of the actuator bearing assembly. Conventional ball bearing assemblies are subject to mechanical limitations which adversely affect their use in today""s high-performance disc drives. More specifically, conventional ball bearing assemblies are subject to metal wear, increased vibrational resonance and friction, and lubricant out gassing. These problems exacerbate the difficulties experienced in selectively positioning a transducer in disc drives with elevated aerial densities.
Electromagnetic bearing assemblies have been proposed as a solution to these problems. An electromagnetic bearing is a device that supports and controls the position of an object immersed in an actively controlled magnetic field. Electromagnetic bearings reap the obvious benefit of having negligible rotor to stator friction and negligible mechanical abrasion.
Magnetic bearings are highly effective for rotatably supporting a rotor which is effectively floated by a controlled magnetic field established by passive (permanent) or active (electromagnetic) magnets located on the stator. Typically, the magnetic field is controlled by a closed-loop feedback system incorporating inductive sensors and variable electromagnets. U.S. Pat. No. 5,111,102 issued May 5, 1992 to Meeks (xe2x80x9cMeeks""102xe2x80x9d) provides thorough explanation of magnetic bearing assemblies and the closed loop feedback control used to maintain the magnetic suspension.
Magnetic actuator bearing assemblies are known in the art of disc drive design and manufacture. For example, U.S. Pat. No. 5,808,839 issued Sep. 18, 1998 to Dunfield et al. (xe2x80x9cDunfield""839xe2x80x9d) discloses a magnetic bearing assembly which is mechanically coupled about its axis. Dunfield""839 fails to make use of complete tri-axial levitation, however, and instead just provides two (X-Y) axis suspension. Further, prior art magnetic bearing assemblies fail to relate the magnetic suspension of the actuator to its servo control. Therefore, there is a pressing need to develop a full-floating actuator bearing assembly which beneficially interrelates the controlled actuator levitation to its complex servo control.
The present invention provides an improved bearing cartridge for pivotally attaching a rotary actuator to the basedeck of a hard disc drive. The improved bearing cartridge also improves servo control of the rotary actuator during seek and track following operations.
In accordance with preferred embodiments, the improved bearing cartridge comprises an active magnetic bearing assembly which three dimensionally suspends the rotary actuator within strictly defined tolerances. The active magnetic bearing assembly preferably comprises a plurality of directional electromagnetic drivers, a plurality of directional permanent magnets and a plurality of sense coils.
The circuitry corresponding to the active magnetic bearing assembly provides a control scheme which enables the active magnetic bearing to detect a disturbance in the actuator and apply corrective current to the proper electromagnetic drivers. The circuitry also provides a feedforward signal indicative of actuator translation to a voice coil motor (VCM) servo control circuit which adjusts the amount of current applied to a VCM used to position the actuator. In another aspect, the VCM servo control circuit feeds forward a seek expectancy signal indicative of an impending seek. Alerting the active magnetic bearing assembly of an upcoming seek operation allows the active magnetic bearing control to prospectively xe2x80x9cbracexe2x80x9d itself for resultant torque forces generated by the pivoting actuator arm.